This invention relates, in general, to an apparatus for measuring physical properties of golf club shafts and, more particularly, to a method and apparatus for measuring a flexural characteristic of a golf club shaft.
In the field of designing, modifying, and fitting golf clubs, it is advantageous to know the physical properties of the golf club shaft as well as the physical properties of the golf club head. It is common in the industry to rate clubs based on the flexural stiffness designated typically by the terms: Extra Stiff (XS); Stiff (S); Firm (F); Regular (R); Average (A); and Ladies (L). The flexural stiffness is important because it determines the maximum bending as well as the first bending mode frequency of the shaft and, therefore by selecting the appropriate shaft stiffness, the club can be optimized for the swing speed of the particular golfer. The torsional stiffness of the golf club is also important because it determines the maximum windup of the club head relative to the shaft and the torsional frequency at which the club head oscillates about the axis of the golf club shaft during the swing. For optimum performance, in addition to matching the flexural stiffness of the shaft to the player""s swing speed, the torsional stiffness of the shaft should also be matched to the club head swing weight and the player""s swing speed. Another important physical property of the golf club shaft is its frequency of oscillation because it provides a reproducible and reliable index of shaft flexibility. Further, the frequency of oscillation allows frequency matching of golf clubs to form a set of golf clubs which have a substantially uniform xe2x80x9cfeelxe2x80x9d to the golfer.
A common prior art apparatus for determining the deflection of a golf club shaft is illustrated in FIG. 1. What is shown in FIG. 1 is a flex board 10 having a golf club shaft clamping structure 11, a plurality of mirrors 12, and a golf club shaft 13. Golf club shaft 13 has a butt end 14, i.e., a grip end, and a tip end 15, i.e., a head end. Clamping structure 11 includes a receptacle 17 for receiving butt end 14 of golf club shaft 13 and a clamp 18 for securing golf club shaft 13 to flex board 10. Flex board 10 further includes calibrated scales 19 along each mirror 12. Calibrated scales 19 are marked in inches and are used to measure the deflection or flexure of golf club shaft 13 when a mass 20 is attached to tip end 15 of golf club shaft 13. Typically, golf club shaft 13 is manually adjusted by an operator after insertion into clamping structure 11 to ensure that it is properly mounted to flex board 10. The operator then adds mass 20 to tip end 15 and records the number of inches golf club shaft 13 is deflected at a predetermined distance along the golf club shaft. For example, the deflection can be measured at the location indicated by reference number 21 or at the location indicated by reference number 22. Although this technique has been used for many years, it is dependent on human operators who may introduce variation into the measurements because of parallax error and operator fatigue. In addition, this technique does not permit making continuous flex measurements along the golf club shaft, i.e., the measurement is made at a discrete point along the golf club shaft.
The present invention provides a method and an apparatus for accurately determining the flexure of stiffness of a shaft by measuring the deflection of the shaft. In a preferred embodiment of an apparatus for measuring the deflection of a golf club shaft incorporating features of the present invention, a clamping mechanism is provided for securing one end of a shaft to a support structure of the apparatus. A flexing mechanism is provided to create a bending moment on the shaft by moving one end of the shaft. In one embodiment, the flexing mechanism comprises a pulley system in which a mass is coupled to the end of the shaft not secured to the support structure. In another embodiment, the flexing mechanism comprises a stop against which the end of the shaft not secured to the support structure is pushed. An image recording device creates an image of the shaft before and after the creation of the bending moment on the shaft. The image is transmitted to a computer which creates a model that mathematically describes the deflection of the golf club shaft. A plot in accordance with the mathematical model can be displayed on a computer screen. The flexure of stiffness of the golf club shaft is determined from the deflection.